1. Technical Field of the Invention
The present invention relates to electromechanical transducer structures and, more particularly, to loudspeaker compression driver and horn structures.
2. Description of the Background Art
Conventional prior art audio frequency reproduction systems use transducers to convert electrical energy to acoustical energy. Systems used for the reinforcement of speech and music are referred to as Sound Reinforcement Systems. These systems are used to reinforce the program material (voice, music or other material) by providing an increase in signal level or gain in order to generate sufficient sound pressure levels in large spaces.
Sound reinforcement systems often use devices known as compression drivers and horns to reinforce the program material. The compression driver is a simple acoustic transducer that uses a small and light weight diaphragm to convert the electrical signals to acoustic signals. The small diaphragm will exhibit fewer resonant modes than a large diaphragm and the lower mass associated with a small diaphragm can produce a higher conversion efficiency.
The small diaphragm, however, has a lower radiation impedance than a larger diaphragm so a horn is coupled to the “exit” of the compression driver. The horn performs an impedance matching function and acts to “transform” the low radiation impedance of the driver to a higher radiation impedance associated with the mouth of the horn radiating into free space. The small entrance of the horn is mated to the small diameter acoustic exit of the compression driver. The acoustic impedance associated with this small area is then transformed to a higher acoustic impedance associated with the larger opening of the horn, referred to as the horn mouth. The rate at which the cross sectional area of the horn changes between the small opening (or throat) and the large opening (or mouth) is referred to as the flare rate.
In addition to acting like an acoustic transformer, the horn also acts to direct the radiated energy in a specific location. The walls of the horn act to guide the radiated wave fronts. In this way the total radiated acoustic power from the driver is concentrated into a portion of space smaller than the space had the horn not been mounted to the driver. The acoustic density, or energy per unit area, is increased and, as a result, the sound pressure level in an area is higher than it would be if the horn were not coupled to the driver for long wavelength conditions (i.e., when the radiated wavelength is long relative to the horn it is referred to as a “long wavelength”). It is a common practice for horns to exhibit circular, elliptical, square, or rectangular radiation patterns.
This horn/driver system has a bandwidth or operating range. The low frequency response of the horn/driver system is limited by the length and mouth area of the horn. When the radiated wavelengths become large compared to the length and mouth circumference, the horn is no longer able to radiate any appreciable acoustic power and the overall horn/driver efficiency is substantially reduced. For the mouth of the horn to have relatively high acoustic impedance, the following relationship must be maintained:ka>1  (1)                where k=(2*pi)/wavelength and a=mouth radius.        
This equation basically requires that mouth circumference (i.e., 2*pi*a) be greater than the wavelength of the lowest frequency to be effectively radiated. This frequency, where the wavelengths become long relative to the mouth circumference, is referred to as the cutoff frequency.
There are many parameters that affect the high frequency response of the compression driver and horn combination. A specific area of interest is the high frequency limit related to the system's ability to maintain the desired directional pattern. A desirable property of a horn is its ability to maintain a specific directional pattern independent of frequency. These horns, are often referred to as “constant directivity” horns (see “What's SO Sacred About Exponential Horns”, Keele, D. B. Audio Engineering Society 51st Convention, May 13-16, 1975). Many sound reinforcement applications require this property for accurate coverage of a specific area.
The ability of a horn to maintain constant directivity is related to the radius of the compression driver exit. As the wavelengths become short compared to the exit radius, the directivity pattern of the wave front emerging from the driver exit is reduced, becoming more narrow. The directivity pattern of the radiated waveform has a main lobe with a width referred to as the beamwidth. The beamwidth is described in terms of an angular extent from a central axis of the horn, where the axial (or “on-axis”) response is measured from the horn's central or major axis. The specific angle is determined by finding the points on either side of the horn's major axis where the sound pressure level (“SPL”) has decreased 6 dB from the SPL measured on axis. (This assumes that the acoustic pressure is a maximum on the horn's central or major axis). The included angle between the −6 dB points is referred to as the beamwidth. If the radiated directivity, or beamwidth, becomes less than the included angles of the horn, then the radiation pattern is no longer constant and the wave front radiated by the driver is no longer controlled by the included angles of the horn. (Reference “On the Radiation of Sound from an Unflanged Circular Pipe”, Levine and Schwinger Physical Review, Vol 73 Number 4, 1948), (“Acoustics”, Beranek, Chapter 4 Radiation on Sound, McGraw-Hill 1954).
FIG. 1 is a cross sectional view of a typical compression driver. A diaphragm mounted to a flexible membrane has an annular coil attached. The annular coil, or voice coil, is suspended in the magnetic gap and the diaphragm is spaced over the phase plug. Acoustic radiation from the diaphragm is transmitted thru the openings in the phasing plug. The phase plug openings may be radially oriented, circumferentially oriented, or a series of simple holes. The summation of the cross sectional areas associated with the phase plug openings forms the acoustic loading of the diaphragm. This phase plug cross sectional area can be made equal to the diaphragm area but is usually substantially lower. The change in cross sectional area between the diaphragm and the phase plug openings is the source of the loading. The volume of air between the diaphragm and the phase plug is compressed due to this reduction in area. The radiation impedance is increased by the square of the ratio of the diaphragm area and the phase plug's initial area.
The individual channels of the phase plug add to an overall area, still smaller than that of the diaphragm area, at the plane defined in FIG. 1 as “plane “A””. Typical compression driver design then includes some linear distance proceeding toward the outlet, or throat of the driver, that expands the cross sectional area in some fashion. This section may be the length defined by the thickness of the magnetic return path backplate. This length is shown in FIG. 1 as the distance between plane “A” and plane “B”. In other common designs an adaptor plate is added to the rear of the magnetic return backplate and is the thickness defined by the distance between plane “B” and plane “C”. The area at plane “B” or plane “C” is always larger than the area of plane “A” in order to not introduce acoustic reflections associated with a reduction in area.
As a consequence of moving farther away from plane “A”, and the necessary increases in cross sectional area, the associated radius at any plane away from the summation point of the phase plug (plane “A”) is increased. This increase in the radius then limits the ability of the driver to produce a wide dispersion and broad radiation pattern as frequency is increased.
There is a need, therefore, for a compression driver motor structure that overcomes these problems and provides a wider dispersion and broader radiation pattern as frequency is increased, for a given horn mouth configuration.